The present invention relates to a multifunction terminal circuit for using a single terminal for a plurality of uses and, more particularly, to the multifunction terminal circuit which is useful to reduce the number of external terminals of an IC package.
Generally, the IC package of the dual in-line type has mounting standards set up in accordance with the number of the external terminals (external pins). Accordingly, a printed circuit board designed for 24 pins, for example, accepts the mounting thereon of a 24-pin type IC made by any maker. The number of pins of dual in-line ICs are usually 16, 24 and 40, for example. The IC package is larger in size as the number of pins thereof is larger. As a result, the price of each IC rises and it is difficult to make small a circuit block using such ICs. Therefore, it is more desirable to employ a 24-pin type IC package than that of a 26-pin type IC package.
Conversely, when the number of pins used is limited, it is desirable to use the external pins as effectively as possible, in order to bring about many functions of the IC package used. The effective use of the external pins is effected by using a proper code conversion and a matrix in an IC memory, for example. In the case of an IC memory model TMM-121C manufactured by Toshiba Corporation in which the inventors serve, the memory contents of 512 words are specified by five row data and four column data. In other words, 512 words are selected by using only 9 pins. In fact, there is a case where, although the external pins are most effectively used by such a circuit technique, the number of the pins must be increased for some function requirements. In such a case, the IC package of 26 pins or more must be used in place of the 24-pin package. However, the increase of the number of pins is unacceptable. Therefore it is desirable to form a new type IC device with an expanded function and a compatibility with the same type IC device previously developed. Even in such a case where no compatibility with the old device must be taken into account, reduction of the number of external pins while retaining satisfactory number of functions is very important for making the size of the IC small and the cost of it low.